W3C home > Mailing lists > Public > www-style@w3.org > January 2000

Re: Units, font sizing, and zoom suggestion for CSS 3

From: Erik van der Poel <erik@netscape.com>
Date: Thu, 20 Jan 2000 12:56:24 -0800
Message-ID: <38877678.DA37CA3B@netscape.com>
To: Karlsson Kent - keka <keka@im.se>
CC: www-style <www-style@w3.org>
Karlsson Kent - keka wrote:
> But the area of typography has never been ruled by strict standards
> and the "em-age" varies.  I once did a web search to find definitions
> of "em" (attached). As you can see the definitions are very varied
> (including equating 1 em and 1 pica).

Sigh. I'm beginning to wonder whether defining font-size in terms of
"em" or "em square" is good enough. TrueType definitely has a field
called "unitsPerEm" in the "head" table:


(This URL is actually for the OpenType spec, but that part is similar to
TrueType, if not identical.)

TrueType is used on Windows, Mac, OS/2, and possibly others. Even Unix
has some support for TrueType.

I would really like to move away from the definition involving the words
"when set solid". That definition just doesn't mean very much, and
that's why I had to ask so many questions on this mailing list.

The em square is defined in CSS2 as follows:

  15.4.3 Coordinate units on the em square

  Certain values, such as width metrics, are expressed in
  units that are relative to an abstract square whose
  height is the intended distance between lines of type
  in the same type size. This square is called the em
  square and it is the design grid on which the glyph
  outlines are defined. The value of this descriptor
  specifies how many units the EM square is divided into.
  Common values are for example 250 (Intellifont), 1000
  (Type 1) and 2048 (TrueType, TrueType GX and OpenType).

  If this value is not specified, it becomes impossible
  to know what any font metrics mean. For example, one
  font has lowercase glyphs of height 450; another has
  smaller ones of height 890! The numbers are actually
  fractions; the first font has 450/1000 and the second
  has 890/2048 which is indeed smaller.

This definition seems to have a mistake. The em square height is not the
intended distance between baselines. You need to add the recommended
leading to the em square height to get the recommended inter-baseline

With em square defined like this (plus my correction), maybe it is OK to
define font-size in terms of em square.

Received on Thursday, 20 January 2000 15:59:10 UTC

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